1. Technical Field
The present invention relates to a camera in which the single frame film advance and shutter cocking are effected by the drive force of a built-in motor, and particularly to a camera provided with a detector device that, once the last frame of the film has been exposed, automatically detects that it is not possible to advance the film beyond the current frame.
2. Description of the Prior Art
Compared with cameras that are constructed such that the film advance and shutter cocking are respectively effected by the drive of separate motors, cameras in which the film advance and shutter cocking are effected by the drive of a single motor are lighter, more compact, and cheaper. In connection with the latter camera construction, the following two systems have been known as the transmission means for transmitting the drive force of the single motor to the film drive section which effects the film advance and to the shutter drive section which effects the shutter cocking. One of them is a system wherein the film drive section and the shutter drive section may be independently linked to a single motor via clutches, and the other is a system wherein the film drive section and the shutter drive section are linked together to a single motor via a transmission mechanism which drives both the film drive section and the shutter drive section.
However, in the former transmission system, as the film drive section and the shutter drive section are independently driven by a single motor, a malfuntion or inconvenience as described below will occur when it becomes impossible to advance the film further. When it becomes impossible to advance the film further, the advancing of the film by the film drive section stops part-way through a cycle, i.e. in an uncompleted state, and the shutter cocking operation by means of the shutter drive section is continued and completed, irrespectively of the state of the film drive section. If the shutter button is pressed depressed down subsequently, the camera shutter mechanism will operate normally, and an exposure will be made, causing multiple exposure of the last frame, or a portion thereof. Accordingly, it is necessary to prohibit the aforementioned operation of the camera's shutter mechanism, to prevent multiple exposure, by detecting when the film advance has become impossible. It may be possible to detect the operative condition of the film drive section mechanically if the film drive section and the shutter drive section are provided with a film advance completion detection member and a shutter cocking completion detection member provided to respectively detect the completion of the respective operations of the film drive section and the shutter drive section, and also with a member which detects the mutual status of the two detection members (i.e. detects whether or not the film advance is completed when the shutter charge has been completed). However, with this arrangement, the camera becomes mechanically complicated and cumbersome.
In the latter transmission system, the film drive section and the shutter drive section are operated together, and the operation of both the film advance and shutter charge operations stop part-way through a cycle, i.e. in the uncompleted state when it becomes impossible to advance the film. Accordingly, this system does not allow the defect of multiple exposure as discussed above with respect to the former type. However, if the camera with the latter transmission system is further provided with a mechanism for rewinding the film by means of the drive of the same single motor in addition to the mechanism for advancing the film and cocking the shutter by the same motor, an additional clutch mechanism is required in association with the single motor to disconnect the film drive section from the shutter drive section, and to drive the film drive section only in the opposite direction in communication with the single motor. This clutch mechanism must be provided in association with the transmission means, and so a camera with such a mechanism becomes complicated. On the other hand, the former transmission system operates the film drive section independently of the shutter drive mechanism, and so the film can be rewound simply by driving the film drive section in reverse without the need for any further clutch mechanism. Thus, both transmission systems have respective advantages and disadvantages.
Besides the mechanical detection means described above, electric detection means are known detect the inability of further film advance in accordance with whether or not the time taken to advance the film has exceeded a prescribed time such as is required for advancing the film, or whether or not the electric current of the film advance motor has exceeded a predetermined boundary level for the detection of the increase of the current when it has become impossible to advance the film. However, the period for the film advance for one frame and the current for driving the motor at the time of normal film advancing, vary in accordance with variations in the power source voltage and the advance load or back tension of the film in use, so that the circuit constants of the electrical circuit must be determined in consideration of the electric factors such as the power source voltage variation and the load current variation which varies in alanlog form. A considerable amount of time is thus consumed in the determination of these circuit constants in the assembly of each camera with such detectors. Thus, it is desired to provide a circuit that can electrically detect the state of the film advancement, with a simple circuit structure that does not require consideration of these factors.